# Alternative translation initiation codons for the plastid maturase MatK: unraveling the pseudogene misconception in the Orchidaceae

## TL;DR

This study shows that the matK gene in orchids is functional despite being labeled a pseudogene, due to an alternative start codon that allows MatK protein expression.

## Contribution

The study reveals a novel alternative translation initiation codon in orchid matK genes, resolving a pseudogene misconception.

## Key findings

- An alternative AUG initiation codon upstream of the consensus site allows functional MatK translation in orchids.
- Immunodetection and reverse-transcription confirm MatK protein expression in orchids previously thought to have a pseudogene.
- Phylogenetic analysis shows the alternative initiation codon evolved de novo within Orchidaceae.

## Abstract

The plastid maturase MatK has been implicated as a possible model for the evolutionary “missing link” between prokaryotic and eukaryotic splicing machinery. This evolutionary implication has sparked investigations concerning the function of this unusual maturase. Intron targets of MatK activity suggest that this is an essential enzyme for plastid function. The matK gene, however, is described as a pseudogene in many photosynthetic orchid species due to presence of premature stop codons in translations, and its high rate of nucleotide and amino acid substitution.

Sequence analysis of the matK gene from orchids identified an out-of-frame alternative AUG initiation codon upstream from the consensus initiation codon used for translation in other angiosperms. We demonstrate translation from the alternative initiation codon generates a conserved MatK reading frame. We confirm that MatK protein is expressed and functions in sample orchids currently described as having a matK pseudogene using immunodetection and reverse-transcription methods. We demonstrate using phylogenetic analysis that this alternative initiation codon emerged de novo within the Orchidaceae, with several reversal events at the basal lineage and deep in orchid history.

These findings suggest a novel evolutionary shift for expression of matK in the Orchidaceae and support the function of MatK as a group II intron maturase in the plastid genome of land plants including the orchids.

The online version of this article (doi:10.1186/s12862-015-0491-1) contains supplementary material, which is available to authorized users.